Mixed field personnel dosimetry method and apparatus for determination of risk related quantity via a nearly tissue equivalent multi-element thermoluminescent dosimeter

ABSTRACT

The method and system interpretation for determining dose equivalents of a multi-element thermoluminescent dosimeter in mixed photon-beta and photon-neutron fields is described. The four TL Elements are covered by nearly tissue equivalent filters including only one metal filter which is used to provide low energy photon discrimination. In a mixed field, deep and shallow dose equivalents and the identity of the field&#39;s components are derived from the dosimeter&#39;s response in pure fields.

RELATED APPLICATION DATA

This application is a continuation-in-part of prior U.S. patentapplication Ser. No. 07/216,343 filed July 7, 1988 and entitled"Personnel TL Dosimetry Method and Apparatus for Determination of RiskRelated Quantity", which application is hereby incorporated herein byreference.

DISCLOSURE

The invention herein described relates generally to thermoluminescencedosimetry and, more particularly, to a method and apparatus for accuratedose equivalent determination and radiation field mixtureidentification.

BACKGROUND OF THE INVENTION

Considerable interest has been generated in recent years in thedevelopment of multi-element thermoluminescent dosimeters and theassociated dose calculation algorithms, especially as applied to largescale routine personnel dosimetry. Many facilities must comply withaccreditation programs such as the Department of Energy LaboratoryAccreditation Program (DOELAP), or the National Voluntary AccreditationProgram (NVIP). Prior dosimeters and associated algorithms haveexperienced difficulties in meeting the criteria set for low energyphotons or mixtures of low energy photons with beta particles and/orneutrons.

RELATED APPLICATION

U.S. patent application Ser. No. 07/216,343 discloses an improvedmulti-element TL dosimeter together with a dose calculation method whichare designed to enable users to meet the ever growing demands of modernpersonnel dosimetry and also environmental monitoring. The thereindescribed methodology provides for improved interpretation of dosimeterresponse in terms of risk related quantity, i.e., dose equivalent. Thedosimeter and method are capable of identifying the mixture type in avariety of mixed fields and estimating the relative contribution ofmajor components. The dosimeter and dose calculation method enable apersonnel dosimetry system to accommodate a wide range of radiationtypes and energies.

More particularly, the dosimeter described in the aforesaid applicationis composed of two parts, namely a TLD card and a holder. The TLD cardincludes multiple thermoluminescent (TL) elements and the holderincludes associated tissue equivalent radiation modifying filters. ThreeTL elements and associated filters function as a basis for shallow doseestimation, a basis for deep dose estimation and as an energyspectrometer for low level energy photons, respectively. The dosimetermay include one or more other TL elements and associated filters forother purposes such as neutron dose estimation. Element CorrectionCoefficients (ECC's) are generated to relate the TL efficiency of eachTL element of an entire dosimeter population (field dosimeters) to themean TL efficiency of a small subset of this population which is usedonly for calibration purposes (calibration dosimeters). When an ECC isapplied to the response of each individual TL element of any dosimeter,its TL efficiency is virtually identical to the mean value of thecalibration dosimeters group.

As disclosed in the aforesaid application, dosimeter response can beused to identify the radiation field mixture composed of beta particlesand/or photons and to determine the relative contribution of eachcomponent in the mixed radiation field. This is accomplished by the useof mixture identification formulas that are based on superposition ofradiation fields and the algorithmization of the response correlationbetween two pairs of TL element response ratios. Mixture identificationcurves (characterizations) are obtained and these are very different forvarious radiation field mixtures and compositions, thereby enablingidentification of mixture type and component contribution.

The aforesaid application also discloses a calibration methodology whichlinks the response of the dosimeter to a variety of different radiationfields calibrated by the National Bureau of Standards or likestandardization agency to the response of the dosimeter to a local anduncalibrated reference source. This involves definition of a localreference or generic unit.

The present invention expands the methodology of the aforesaidapplication by providing for accurate interpretation of the dosimeterreading in terms of risk related quantity for neutron fields andmixtures thereof with photon and/or beta fields.

SUMMARY OF THE INVENTION

The present invention provides improvements in a dosimetry method andsystem which provide for improved radiation monitoring. Improvements aremade in interpretation of the response of a dosimeter to mixed radiationfields including neutrons in mixture with beta and/or gamma fields.

According to one particular aspect of the invention, a dosimetry methodcomprises using a dosimeter to monitor exposure to a mixed radiationfield composed of photons and neutrons, the dosimeter including firstand second elements sensitive to photons and insensitive to neutrons anda third element sensitive to photons and neutrons, and first and secondfilters for said first and second elements having different photonattenuation characteristics and a third filter for the third elementhaving a photon attenuation characteristic different from the neutronattenuation characteristic of the first filter; heating the radiateddosimeter to obtain thermoluminescence emission of the elements; andquantitatively relating the emission to the radiation incident on thedosimeter, said quantitatively relating step including determining theneutron component of incident radiation by taking the difference betweenthe response of the first and third elements with one of the responsesbeing weighted as a function of photon energy.

According to other aspects of the invention, provision is made toaccount for fading effects and to account for supralinearity ofLiF:Mg,Ti.

The foregoing and other features of the invention are hereinafter fullydescribed, the following description and the annexed drawings settingforth in detail a certain illustrative embodiment of the invention, thisbeing indicative, however, of but one of the ways in which theprinciples of the invention may be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

In the annexed drawings,

FIG. 1 is a plan view of a personnel beta-gamma dosimeter havingpreferred usage in the practice of the invention;

FIG. 2 is a sectional view of the dosimeter taken along the line 2--2 ofFIG. 1;

FIG. 3 is a sectional view of the dosimeter taken along the line 3--3 ofFIG. 1;

FIG. 4 is an illustration of a variety of possible element responseratios for a single ratio (L3/L1);

FIG. 5 is a graphical illustration showing mixture identification curves(MIC's) for low energy photons and high energy beta mixed radiationfields;

FIG. 6 is a graphical illustration showing mixture identification curvesfor low and high energy photons and for low energy photons and lowenergy beta rays;

FIG. 7 is a graphical illustration showing mixture identification curvesfor high energy photons and low energy beta mixed radiation fields;

FIG. 8 is an illustration of algorithm verification results;

FIG. 9 is a diagrammatic illustration of a personnel dosimetry systemaccording to the present invention;

FIG. 10 is a graphical illustration of experimental data used to obtaina neutron weighting function for mixed photon-moderated CF-257 neutronfields; and

FIG. 11 is an illustration of test results where the empty barsrepresent shallow dose, the full bars represent deep dose, the solidhorizontal straight lines represent current DOELAP tolerance levels, andthe dotted line represents the expected new DOELAP tolerance levels.

DETAILED DESCRIPTION

In FIGS. 1-3, the dosimeter 10 can be seen to be principally composed oftwo parts, a thermoluminescence dosimetry (TLD) card 11 and a holder 12which carries radiation modifying filters 13-16. The TLD card 11consists of four LiF:Mg,Ti thermoluminescence (TL) elements 17-20 ofdifferent thicknesses and compositions mounted in known manner betweentwo Teflon (PTFE) sheets on an aluminum substrate. Each TL element iscovered by its own unique filter which provides different radiationabsorption thicknesses to allow dose estimation for various organs inrisk. The TL elements 17-20 are located in positions designated 1-4,respectively, and the card 11 may be sealed in the holder 12 by annularseal 21.

In the illustrated preferred dosimeter, TL element 19 in position 3 is athin solid TLD-700 phosphor (Lithium-7 enriched fluoride) having apreferred thickness of 0.09 mm. This element is protected from theenvironment by filter 15 which is a thin aluminized Mylar sheet having apreferred thickness of 0.0025 inch and a tissue equivalency of 7.5mg/cm². The thin aluminized Mylar sheet corresponds to an open window inthe holder. The response of element 19 provides a basis for shallow doseestimation. As a result of its small thickness (a factor of 10 thinnerthan a heretofore standard 3 mm by 3 mm by 0.9 mm TLD ribbon),underestimation of shallow dose contribution of low energy beta rays isreduced. The small thickness of both the element (substantially lessthan 0.9 mm) and filter (substantially less than 0.9 mm) provide forreduced energy dependent response to low energy beta particles.

TL elements 17 and 18 in positions 1 and 2 each are a thicker TLD-700phosphor having a preferred thickness of 0.4 mm. Element 17 is coveredby filter 13 to provide a basis for deep dose estimation. The filter 13has a density thickness substantially greater than 250 mg/cm² andpreferably 1000 mg/cm² to minimize the contribution of high energy betarays (Sr/Y-90) to the deep dose response and to provide tissueequivalent absorption thickness as close as possible to the actual depthin tissue where deep dose estimation is desired. This results in smallercorrection factors to be applied to the response of the TL element inposition 1 when the deep dose index is estimated.

Element 18 is covered by filter 14 including a copper filter element 23,as illustrated. The variation with energy of the photon attenuationcharacteristics of the copper filter element 23 located in position 2gives the dosimeter the ability to act as a crude energy spectrometerfor low energy photons.

TL element 20 in position 4 is a neutron sensitive, TLD-600 phosphor(Lithium-6 enriched fluoride) having a preferred thickness of 0.4 mm.This element is shielded by a 300 mg/cm² tissue equivalent filter toenable dose estimation to the lens of the eye and to measure neutrondose in the absence of thermal neutrons. TLD-600 and TLD-700 typephosphors are available from Harshaw Crystal & Electronic Products,Solon, Ohio.

It is noted that the thick (0.4 mm) TL components of dosimeter 10 areless than a heretofore standard thickness of 0.9 mm by at least about afactor of 2. It also is noted that nearly tissue equivalent TL elementsand tissue equivalent filters are used. A plastic filtration of 1000mg/cm² for deep dose estimation is preferred over lead filters whichheretofore have been used.

As in conventional personnel dosimetry systems, the dosimeters are wornby personnel so that the dosimeters will be exposed to the same amountand type of radiation. On a periodic basis such as daily the dosimetersare read by a TLD card reader. A preferred TLD card reader is a Model8800 automatic TLD card reader sold by Harshaw Crystal & ElectronicProducts, Solon, Ohio. The Model 8800 TLD card reader utilizes anon-contact heating technique based on pre-purified hot nitrogen gas.The Model 8800 TLD card reader employs a programmable, preciselycontrolled linear time temperature profile for reproducible heating ofthe dosimeter elements. This is desirable because the amount ofradiation induced TL is dependent on the thermal history of the materialas well as on the heating rate during readout. A fully controlledheating cycle is therefore important especially for low dosemeasurements. Various aspects of the Model 8800 TLD card reader aredisclosed in U.S. Pat. No(s). 4,727,253 and 4,835,388.

For the most part TL elements can not all be manufactured to haveexactly the same TL efficiency [TL efficiency (TLE) is defined as theemitted TL light intensity per unit of absorbed dose]. In view of this,individual element correction coefficients (ECC's) preferably aredefined, developed and applied.

A batch of TL elements typically has variation in TL efficiencies of10-15% (one relative standard deviation). This spread can be virtuallyreduced to 1-2% if ECC's are applied. Generation of ECC's is based onrelating the TL efficiency of each TL element of the entire dosimeterpopulation, i.e., the field dosimeters, to the mean TL efficiency of asmall subset of this population which is used only for calibrationpurposes, i.e., calibration dosimeters. When the ECC is applied to theresponse of each individual TL element of any of the field orcalibration dosimeters, its TL efficiency is virtually identical to themean value of the calibration dosimeters group and as a result all theTL elements have ideally the same TL efficiency. For a detaileddiscussion of the general concept of element correction coefficients,reference may be had to Plato et al, "Production of Element CorrectionFactors for Thermoluminescent Dosimeters", Health Physics, 49, 873-881(1985).

As used herein, TL response (TLR) of a specific TL element is defined asthe measured quantity which results from subjecting the dosimeter to oneunit of a given ionizing radiation. The units in which the measuredquantity is expressed depends on the means which are used to detect theemitted TL photons. If every single photon emitted by the TL elementcould be counted and the units of the ionizing radiation expressed interms of dose, the TL response would be equal to the TL efficiency.Normally this is not the case and the measured quantity is eitherexpressed in units of charge or in units of counts for chargeintegration and photon counting TL measuring techniques, respectively.For background information, reference may be had to Spanne, "TL ReadoutInstrumentation", Thermoluminescence and Thermoluminescent Dosimetry,Vol. III, Ch. 1, CRC Press, Orlando, FL (1984). Irrespective of themethod of measurement which is being used, the TL response of a elementin position i will always be proportional to the TL efficiency; that is:

    TLR.sub.i =k.sub.i TLE                                     (1)

Where K_(i) is the proportionality constant. The proportionalityconstant k_(i) does not have to be identical from one position toanother due to different photomultiplier gains, for example.

The calibration dosimeters may all be subjected to a quantity L ofionizing radiation from a given source where L can be expressed in anyconvenient units. For example, the quantity L may be expressed in unitsof time of irradiation, providing that for each dosimeter the geometryrelative to the source is kept constant and the radiation field ispenetrating enough to deposit energy in the entire dosimetric volume.Since the measured TL effect is the sum over the entire sensitive volumeof the TL element, the energy deposition profile does not have to beuniform as long as it is identical for each dosimeter.

By letting ECC_(ij) be the element correction coefficient for element iin card j (i=1,2,3,4 and j=1,2,. . . ,1 where 1 is the number ofcalibration cards), and letting TLE_(ij) and TLR_(ij) be the TLefficiency and the TL response of element ij, respectively, then thefollowing equation can then be defined:

    ECC.sub.ij =TLE>.sub.i /TLE.sub.ij (2)

when ##EQU1## where "<>" is used to denote the average value. With theaid of Equation (1), Equations (2) and (3) can be written in the form:

    ECC.sub.ij =<TLR>.sub.i /TLR.sub.ij                        (4) ##EQU2##

An implicit assumption in writing Equations (4) and (5) is that the TLDreader response to TL photons did not change during the measurement ofthe entire population of the calibration cards, i.e., k_(i) remainsconstant during the entire TL readout process. Since this step isimportant to the success of generating true ECC's, it is important toperform it in a relatively short period of time and to ensure thestability of the light detection and the heating subsystems by frequentreference light measurements and glow curve analysis to ensure completereadout.

It is convenient to express Equations (4) and (5) in terms of thequantity which is reported by the TLD reader (charge or

counts). If Q_(ij) is defined to be the charge reported by the readerfor element ij following its subjection to a quantity L of ionizingradiation, TLR_(ij) and <TLR>_(i) can be written in the form:

    TLR.sub.ij =Q.sub.ij /n                                    (6)

and

    <TLR>.sub.i =<Q>.sub.i /n                                  (7)

when ##EQU3## Using Equations (6), (7) and (8), Equation (4) can bewritten in the form:

    ECC.sub.ij =<Q>.sub.i /Q.sub.ij                            (9)

when <Q>_(i) is given by Equation (8).

Once ECC's have been established for the calibration cards, each of themhas virtually the same TL efficiency and any statistically significantsubset of calibration cards can be used to generate ECC's for the fieldcards.

Of course a TLD reader for various reasons may change its response to TLphotons since the time that <Q>_(i) was generated. For example, thischange may result from an intentional or accidental change in the highvoltage power supply setting, replacement of damaged photomultipliertubes, replacement of infrared (IR) filters or accumulation of dirt onthe IR filters. If the response of the reader has changed by a factor ofC_(i) for each corresponding position (i32 1,2,3,4) in each TLD card,which change will equally affect the TL elements, the following equationcan be written:

    <Q>'.sub.i =C.sub.i <Q>.sub.i                              (10)

and

    q'.sub.ij =C.sub.i q.sub.ij                                (11)

where q_(ij) is the charge that would have been reported by the readerfor element i of field card j at the time that <Q>_(i) was generated,<Q>'_(i) is the average reported charge from the calibration cards whichwere exposed and read together with the field cards whose elementcorrection coefficients have to be generated, and q'_(ij) is the chargereported by the reader for element i of field card j following itssubjection to quantity L of ionizing radiation. The foregoing assumesthat the subset of calibration cards were also exposed at the same timeand read together with the field cards.

Similar to Equation (9), the element correction coefficients ecc_(ij)for field cards can be defined as follows:

    ecc.sub.ij =<Q>'.sub.i /q'.sub.ij                          (12)

It is noted that the same ecc values for field cards would have beenobtained if they were generated at the time that the ECC's for thecalibration cards were generated or at any other time, since the C_(i)values from Equations (10) and (11) would be canceled out in Equation(12). Once the ecc's for the field cards have been generated and the eccis applied, its TL efficiency is virtually equal to the mean TLefficiency of the calibration cards and as a result, all the cardpopulation will have virtually the same TL efficiency. When new cardsare purchased or added, their TL efficiency can be virtually set to beequal to the existing card population by generating ecc's for the newcards. The only parameter which need remain constant is the inherentsensitivity of the calibration cards which are being used. Moreover, TLDcards of the above described type read by the above identified readercan be subjected to hundreds of reuse cycles without any noticeablechange in their TL efficiency.

The radiation source that is used for generating the element correctioncoefficients for the field cards does not have to be the one used forgenerating the ECC's for the calibration cards provided that a subset ofcalibration cards are subjected to the same radiation field as the fieldcards whose ecc's are generated. Moreover, there is no need to irradiatethe TLD cards mounted in their holders since the only purpose of thisirradiation is to induce an excitation in the TL material which willresult in a measurable TL signal that is proportional to the TLefficiency of the TL element. The interpretation of the dosimeterreading in terms of absorbed dose or dose equivalent is hereinafterdiscussed.

The TL elements under controlled operational conditions will not changetheir TL efficiency and the irradiation geometry can be easilymaintained given that radiation sources generally are stable, or atleast it is relatively easy to apply correction factors to account forthe decay of the radioactive material. Normally the only part of thesystem that will not remain sufficiently stable over long periods oftime or in which something can go wrong is the TLD reader. To maintain aknown relationship between the ability of the reader to convert storedTL information to measurable electric signals (charge or counts) byheating the TL elements and detecting the emitted light, the ratiobetween the TL response of the calibration cards and the deliveredradiation quantity L is expressed in terms of one variable. Since thenumerical value of this variable will be mainly dependent on thecondition of the reader at a given date and time, it is appropriate tocall this variable a reader calibration factor (RCF). As shown elsewhereherein, the value of the RCF although not expressed yet in terms of"real" dose units provides the main link between the TL response interms of charge or counts and the absorbed dose or dose equivalent interms of rad or rem.

The reader calibration factor for position i, RCF_(i), can be defined asfollows:

    RCF.sub.i =<Q>.sub.i /L                                    (13)

where <Q>_(i) is the TL response of a set of calibration cards exposedto a known quantity of radiation L. As discussed above, the radiationquantity L can be expressed in any convenient units. Accordingly, theunit gU (generic unit) can be used as the unit in which the quantity Lis expressed. For example, 1 gU can be equal to the amount ofirradiation delivered during a period of one second by a specific sourcewith a specific geometry to a dosimeter located at a specific distancefrom the source. Since the definition of the unit gU is somewhatarbitrary, once defined for a specific source and geometry, it will havemeaning only for such source and geometry. This source and geometryherein will be called the local source or reference source.

The gU has some similarity to conventional units in the sense that gU isthe unit of the quantity L in a similar way as the Roentgen (R) is theunit of exposure and the rad and the rem are the units of absorbed doseand dose equivalent, respectively. However, unlike the conventionalunits which have universal meaning, i.e., 1 R in California representsthe same amount of radiation as 1 R in Mexico City. In comparison, theamount of radiation which corresponds to 1 gU is completely arbitraryand depends on the way in which one chooses to define his own gU. Thelink between gU and rad or rem is established in the below discussedmanner inasmuch as the purpose of a dosimetry system is to enable themeasurement of absorbed dose or dose equivalent.

To obtain a meaningful RCF, it is necessary to accurately reproduce theirradiations of the calibration dosimeters. One way to do this is to useperiodically calibrated and NBS (National Bureau of Standards) traceablesources which are usually located at special testing or calibrationlaboratories. In this case the quantity L will be the exposure orwhatever quantity the source is calibrated for, and the gU will be theRoentgen (R) or any other corresponding unit. Besides the inconveniencethat this method creates from the point of view of the time, expense,planning, the danger of damage or exposure of the dosimeters duringshipping, and the inability to expose dosimeters in a short notice whena new RCF has to be generated (a PMT tube has been replaced forexample), this approach does not provide much advantage over the use ofa local reference source for generating the RCF, since the RCF is arelative quantity.

Returning to Equation (12), the ecc's for the field cards can beexpressed in terms of the RCF using Equation (13):

    ecc.sub.ij =(RCF.sub.i /q.sub.ij)L                         (14)

From Equation (14), L can be expressed in terms of the RCF, ecc and q:

    L =(q.sub.ij ecc.sub.ij)/RCF.sub.i                         (15)

Once the ECC's and ecc's for calibration cards and field cards have beengenerated, respectively, and the local unit gU has been defined, thelink to an NBS calibrated source located at the calibration laboratorycan now be established.

The calibration laboratory is able to perform the irradiations andreport the delivered quantity in terms of shallow dose and deep dose. Asubset of the calibration cards in their holders (or any other set ofcards which have ECC's) is exposed to Hs and Hd, shallow and deep dose,respectively, from say a Cs-137 calibrated source and read out. Thereader will report its findings in units of gU using Equation (15);however, since the values of Hs and Hd as reported by the calibrationlaboratory are known, one can establish the following relations betweenthe local units gU and the "risk related quantity" units, rem:

    LHs=L/Hs                                                   (16)

for the shallow dose conversion factor, and

    LHd=L/Hd                                                   (17)

for the deep dose conversion factor.

Both LHs and LHd are expressed in units of gU/rem and provide the linkbetween the local source and the NBS calibration standard, in a similarway that the RCF value provides the link between the internal units ofthe reader (counts or charge) and the local source. For chargeintegration systems, the RCF is expressed in units of nC/gU and thequantities RCF*LHs and RCF*LHd are expressed in terms of nC/rem andprovide the link between the internal units of the reader and theshallow and the deep doses. Through routine calibration of the TLDreader directly in terms of "nC/deep rem" and "nC/shallow rem", therewould be no need to establish the relations (16) and (17). Since thedefinition of the gU unit is based on exposing some calibration cards tothe local source following a reproducible procedure, the time intervalsbetween preparation and irradiation and between irradiation and readoutis not important as long as it is kept constant or, if not, fading iseliminated by removing the low temperature peaks.

The time interval between irradiation at the calibration laboratory andreadout does not have to be the same as the time interval used forgenerating the RCF. The reason for this is that the gU is defined for aspecific time between irradiation and readout and as long as this timeis kept constant the definition of gU will not change. The only casewhen fading corrections have to be applied or the low temperature peakshave to be removed would normally be when one wants to apply the valuesof LHs and LHd to calculate the reported dose from a field dosimeter. Inthis case, the differences in the fading during the time intervalsbetween the field irradiation and readout and the NBS calibrationirradiation and readout have to be corrected or, alternatively, the lowtemperature peaks have to be removed.

As used herein, "internal calibration" shall mean the proceduresrequired to maintain and monitor the stability of the TLD reader bygenerating a Reader Calibration Factor (RCF) and to correct forvariations in the TL sensitivity within a batch of TL elements byapplying Element Correction Coefficients (ECC's).

Since the dosimeter may respond differently to different types ofradiation fields or various mixtures (different gU/rem values), thedosimeter response is experimentally characterized and the results ofthis characterization is used in the interpretation of dosimeterreadings for unknown dose and radiation field combinations. In order tocalculate specific dose equivalent values, knowledge of the type ofradiation field or mixture that the dosimeter was subjected to isneeded. The following procedure is provided for identifying theradiation field using the dosimeter readings.

Direct information from the dosimeter reading which is available fordetermining the radiation field type includes the L values from thedifferent dosimeter positions, i.e., L₁, L₂, L₃ and L₄ [see Equation(15)]. The TL element in position 4 is sensitive to neutrons and isreserved for applications involving neutron fields. The remaining threeelements at positions 1, 2 and 3 form two independent ratios L3/L1andL3/L2. The ratio L3/L1as a function of the ratio L3/L2 can berepresented as a function f(x), i.e.,

    x=L3/L2                                                    (18)

and

    f(x)=L3/L1                                                 (19)

The shape of this function and how fast it changes for differentenergies and compositions is important to the dosimeter's ability todiscriminate different radiation fields and/or to determine the relativecontribution of components in mixed fields.

For a mixture of two radiation fields "a" and "b", the response of eachTL element is assumed to be the weighted sum or superposition of itsindividual response to fields "a" or "b" as if the other field would notexist. This assumption means that there is no interaction between theinduced TL effects when the dosimeter is subjected to two or moredifferent radiation fields. This assumption is generally valid althoughthere has been reported in the literature evidence that in some casesthe TL response resulting from mixtures of radiation fields may not beadditive, such as in the case of fast neutrons where a decrease of 10%in the gamma TL signal has been observed as a result of the tendency offast neutrons to release the stored gamma induced signal from previousor simultaneous gamma irradiation. However, if non-additive effectsexist for mixed beta and gamma fields they are expected to be small anddata has shown that this assumption of superposition of radiation fieldsis valid to within a few percent.

This superposition principle can be applied to determine f(x) asfollows. First, N represents the relative contribution of field "a" tothe mixed field and assuming that only two fields exist, 1-N will be therelative contribution of field "b". If the delivered quantities areexpressed in terms of Roentgen or rad in air, N and 1-N will be theweighting factor assigned to each field (for example, if irradiation iseffected such that there are 4 rads in air from field "a" and 2 radsfrom field "b", then N =0.67 and 1-N =0.33). It is noted that thesedelivered quantities are in air and that they are correlated to thedelivered shallow and deep dose using C_(x),s and C_(x),d values,respectively. [Cx,s and Cx,d are exposure-dose-equivalent conversionfactors which relate the exposure in air to the dose equivalent at aspecified depth in a material of specified geometry and composition, inthe instant case for shallow and deep dose.]The relative

response, a_(i), b_(i) (i=1 to 4), of each element to pure field "a" or"b" is defined to be the response of the particular element in units ofgU per unit of delivered dose in air when only one field is being used.

Using the above superposition principle, the relative response ab_(i) ofelement i to a mixture of fields "a" and "b" is written as follows:

    ab.sub.i =Na.sub.i +(1-N)b.sub.i                           (20)

Then with the aid of Equation (20) any one of the L3/L1 and L3/L2 ratiosin a mixed field can be expressed in terms of the relative response ofeach of the elements to the pure fields:

    L3/L1=ab.sub.3 /ab.sub.1 =[Na.sub.3 +(1-N)b.sub.3 ]/[Na.sub.1 +(1-N)b.sub.1 ]                                                         (21)

and similarly,

    L3/L2=ab.sub.3 /ab.sub.2 =[Na.sub.3 +(1-N)b.sub.3 ]/[Na.sub.2 +(1-N)b.sub.2 ]                                                         (22)

Using x for L3/L2, Equation (22) can be rewritten in the form:

    N=[b.sub.3 -xb.sub.2 ]/[x(a.sub.2 -b.sub.2)-(a.sub.3 -b.sub.3)](23)

Substituting N from Equation (23) into Equation (21) and using f(x) forL3/L1, Equation (21) can be rewritten in the form:

    f(x)=[b.sub.3 a.sub.2 -a.sub.3 b.sub.2] x/[(b.sub.1 a.sub.2 -a.sub.1 b.sub.2)x+(a.sub.1 b.sub.3 -b.sub.1 a.sub.3)]             (24)

Equation (24) is used to identify the mixture type in the followingmanner.

The calibration constants a_(i) and b_(i) all are determined once byperforming calibration irradiations at the NBS or NBS traceablecalibration laboratory for all the possible radiation fields of interest(all the possible a's and b's), which may be used as model fields tosimulate possible different responses of the dosimeter to variousradiation fields which may occur in the field. Once those calibrationconstants are known, the value of x (the ratio L3/L2) is computed fromthe response of the dosimeter. f(x) is then computed for this particularx and for all possible radiation field mixtures "a" and "b".

The next step is to compare the measured L3/L1 value to all thecalculated f(x) (all the possible computed L3/L1 ratios for theparticular measured L3/L2 ratio) and select the one which provides thesmaller percentage deviation between the measured and the computed L3/L2ratios to represent the required type of radiation fields mixture, i.e.,the identity of "a" and "b". Once "a" and "b" have been identified,Equation (23) can be used to calculate the relative contribution of eachcomponent, N and 1-N for fields "a" and "b", respectively. If none ofthe computed L3/L1 ratios is in reasonable agreement with the measuredone, the reading should be considered questionable and the dose valueshave to be otherwise assigned such as manually. Such lack of agreement,for example, may result from radiation fields which are different fromthose that were covered by the calibration, or the dose may have beentoo low to provide meaningful measure of the value of N. As will beappreciated, the dose may be calculated based on the average value ofthe calibration constants which are tabulated in Table 2.

In the foregoing manner the components in a mixed field can beidentified and the relative contribution of each component can bedetermined. Once the components and their relative contributions areknown, the deep and the shallow dose in a mixed field can be computed.

Deep and shallow dose are determined from the TL response in units ofgU, R₁ and R₃ for TL elements 1 and 3, respectively. From thecalibration run at the calibration laboratory, deep and shallow dosecalibration values for each of the pure fields can be computed in thefollowing manner.

First let Ra₁ be the response of the element 1 in units of gU when thedosimeter is exposed to d rem of deep dose and Ra₃ be the response ofelement 3 in units of gU when the dosimeter is exposed to s rem ofshallow dose using pure field "a" as reported by the calibrationlaboratory. Similarly, the variables Rb₁ and Rb₃ are defined for purefield "b". Also, pure field calibration values, r_(a1) and r_(a3), interms of gU per deep rem and gU per shallow rem for field "a", aredefined as follows:

    r.sub.a1 =R.sub.a1 /d[gU/rem]                              (25)

and

    r.sub.a3 =Ra.sub.3 /s[gU/rem]                              (26)

Similarly rb₁ and r_(b3) are defined for pure field "b":

    r.sub.b1 =Rb.sub.1 /d[gU/rem]                              (27)

and

    r.sub.b3 =Rb.sub.3 /s[gU/rem]                              (28)

In accordance with the above discussed superposition principle, mixedfield calibration factors r_(ab1) and r_(ab3) for deep and shallow dose,respectively, can be defined to be:

    r.sub.ab1 =Nr.sub.a1 +(1-N)r.sub.b1 [gU/rem]               (29)

and

    r.sub.ab3 =Nr.sub.a3 +(1-N)r.sub.b3 [gU/rem]               (30)

Finally, deep and shallow dose can be computed from the TL response inunits of gU, R₁ and R₃ for elements 1 and 3, respectively, and usingEquations (29) and (30) as follows:

    DEEP DOSE=R.sub.1 /r.sub.ab1 [rem]                         (31)

and

    SHALLOW DOSE=R.sub.3 /r.sub.ab3 [rem]                      (32)

An application of the above method to experimental results will now bedescribed in detail in respect of mixed beta-photo fields.

145 dosimeters were supplied in a single batch to a calibrationlaboratory to be irradiated to various qualities and quantities ofradiation from various beta and gamma fields. Each dosimeter had aserial number that uniquely identified the dosimeter and specified theradiation field to which the dosimeter was to be exposed according to apredetermined irradiation plan. 15 shipping control and replacementdosimeters were also included. The irradiation procedures followed theDepartment of Energy Standard for the Performance Testing of PersonnelDosimetry Systems [DOE/EH-0027 (1986)].

40 dosimeters in groups of 5 were exposed to 8 different pure radiationfields as specified in DOE/EH-0027 and summarized in Table 1. Theresponse of those dosimeters were used to generate the variouscalibration factors as above described. 10 other dosimeters were exposedto the accident categories (I and II) and their response is used to testsupralinearity corrections.

                  TABLE 1                                                         ______________________________________                                        Calibration Irradiations                                                      Radiation Field         Energy                                                ______________________________________                                        1    x-ray NBS filtered Technique - M30                                                                    20 keV                                           2    x-ray NBS filtered Technique - S60                                                                    36 keV                                           3    x-ray NBS filtered Technique - M150                                                                   70 keV                                           4    x-ray NBS filtered Technique - H150                                                                   120 keV                                          5    Gamma Cs-137            662 keV                                          6    Beta (Point geometry) Tl-204                                                                          760 keV (max)                                    7    Beta (Point geometry) Sr/Y-90                                                                        2300 keV (max)                                    8.   Beta (Slab geometry) Uranium                                                                         2300 keV (max)                                    ______________________________________                                    

The remaining 95 dosimeters were irradiated using 19 different mixturesof photons and beta rays and their response together with the responseof the dosimeters that were exposed to the pure fields were used to testthe above described dose calculation algorithm.

The dose levels used to calibrate the system were relatively high,420-2000 mrem for the total deep dose; therefore, no environmentalbackground was subtracted and, additionally, no fading corrections wereapplied. The following results that are presented were all obtainedusing "raw data" and therefore represent the worst possible case fromthe point of view of the lack of background or fading corrections.

50 cards from a different batch were used as calibration dosimeters andwere treated in the above described manner. 10 of these cards were usedto generate element correction coefficients for the dosimeters whichwere sent to the calibration laboratory and which were treated as fieldcards.

The unit gU was defined as the amount of irradiation delivered during aperiod of 0.1 seconds by the internal Sr/Y-90 irradiator employed in theTLD reader. The reference cards were prepared immediately beforeirradiation and readout 30 minutes following the end of the irradiationperiod. Due to the small periods of time involved, care was taken topreserve the order of the cards during all of the irradiation andreadout steps (i.e., the cards were readout in the same order that theywere exposed). Since this procedure usually should involve a relativelysmall number of cards (3 to 10), it is relatively easy to maintain theirorder. When dealing with a larger number of cards, it would be a goodpractice to eliminate the low temperature peaks or to wait a longerperiod of time, such that the time elapsed between irradiation andreadout will be significantly larger than the irradiation time of thecomplete batch. For example, if the irradiation time is 2 hours a periodof 12 hours will be adequate since most of the fast fading component,peak 2, will disappear. In cases like that, it is not important (andalso not practical) to try to keep the cards in order.

The TL response of the calibration cards was used as a measure of theamount of gU units that were delivered. It is appropriate, therefore, tolink the particular irradiation and readout parameters to the gUdefinition.

Before estimating the deep and shallow dose for the mixture categories,the calibration factors r_(ai) and a_(i) were calculated using theresponse of the dosimeters to the pure fields, and the delivered deepdose, shallow dose and delivered exposure or dose supplied by thecalibration laboratory. The results of this calibration are shown inTables 2 and 3 for r_(ai) a_(i), respectively.

                  TABLE 2                                                         ______________________________________                                        Pure Field Calibration Factors - gU/rem                                       Source   Deep Dose - r.sub.al                                                                          Shallow Dose - r.sub.a3                              ______________________________________                                        1   M30      693.0 + - 11.1                                                                            (1.6%)                                                                              837.6 + - 19.7                                                                          (2.4%)                               2   S60      942.9 + - 15.1                                                                            (1.6%)                                                                              1008.1 + - 22.0                                                                         (2.2%)                               3   M150     805.4 + - 16.3                                                                            (2.0%)                                                                              848.1 + - 10.9                                                                          (1.3%)                               4   H150     726.5 + - 39.6                                                                            (5.5%)                                                                              737.2 + - 20.3                                                                          (2.8%)                               5   Cs-137   659.8 + - 14.5                                                                            (2.2%)                                                                              667.7 + - 11.7                                                                          (1.8%)                               6   Tl-204   --              498.0 + - 7.6                                                                           (1.5%)                                 7   Sr/Y-90  --              716.0 + - 10.9                                                                          (1.5%)                                 8   Uranium  --              408.6 + - 10.1                                                                          (2.5%)                                 ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Pure Field Relative response - gU/R or gU/rad                                 Source       a.sub.1 a.sub.2   a.sub.3                                                                             a.sub.4                                  ______________________________________                                        1     M30         311.8   95.6   904.6 736.1                                  2     S60        1008.8  820.7   1159.4                                                                              1191.5                                 3     M150       1183.8  1220.0  1195.6                                                                              1268.3                                 4     H150       1024.2  1073.5  1039.4                                                                              1103.9                                 5     Cs-137      679.5  681.1   687.6 723.4                                  6     Tl-204       2.9    2.7    497.9  5.0                                   7     Sr/Y-90     31.2   157.9   696.0 241.0                                  8     Uranium     22.9    64.6   408.6  97.7                                  ______________________________________                                    

Each calibration factor was computed averaging the response of 5dosimeters which were exposed simultaneously to the same radiationfield. The uncertainties shown in Table 2 represent one standarddeviation of the average. The percentage standard deviation is given inparentheses and, with the exception of one case where its value is 5.5%,all the other values lie in the range of 1.3-2.8%. Considering the factthat the determination of these calibration factors were a result of twocomplete sets of independent measurements, i.e., ecc generation usingthe local reference source and then performing the calibrationirradiation at the calibration laboratory, these figures representexcellent precision of the entire system, i.e., dosimeters (cards andholders), TLD reader and irradiation facilities.

The a_(i) values tabulated in Table 3 were measured with a similarprecision (error is not shown).

The variety of possible L3/L1 ratios for different field mixtures andcompositions are shown in FIG. 4. This diagram, which is not scaled andin which only the extremes are marked, illustrates that only a limitedamount of information related to the mixture composition can be derivedfrom the response of only 2 TL elements (one ratio). A similar diagram(not shown here) could be drawn for the L3/L2 ratio. The L3/L1 ratiosfor the higher energy photon fields, M150, H150 and Cs-137, are allidentical and equal to approximately 1.00 within the experimental errorwhich is 2-3%.

However, when the L3/L1 ratio is plotted as a function of L3/L2, the"degeneracy" shown in FIG. 4 is removed and each mixture type isidentified by its own unique pattern which herein is called the mixtureidentification curve (MIC). Typical results for various mixtures areshown in FIGS. 5, 6 and 7 which illustrate a family of curves calculatedusing Equation (24) and the calibration parameters tabulated in Table 3(a₄ is not being used for photon-beta fields and is reserved forirradiations involving neutrons).

FIG. 5 overlays the MIC's for the mixture of low energy photons (M30)with either Sr/Y-90 or depleted uranium (DU). Both curves start from thesame point where only M30 is present in the mixture and then begin todiverge as the beta component contribution is increased. The symbolsrepresent values calculated using Equation (24) and correspond tovarious N values starting at N=1.00 where only low energy photons arepresent and then continuously decreasing by steps of 0.05 down to N=0.00where only beta particles are present in the mixture. The continuouslines are third order polynomial regressions and were plotted to guidethe eye.

The discrimination ability of this method is further demonstrated inFIG. 6 where MIC's for mixtures of high energy photons with low energyphotons and low energy photons with Tl-204 beta rays are plotted. Ofparticular interest are the mixtures of S60 with either Cs-137 or Tl-204where one curve starts at the same point that the other is terminated.Accordingly, the mixture could be identified utilizing only one of theelement ratios (either L3/L1 or L3/L2 ), but this will occur onlyrarely.

FIG. 7 shows MIC's for mixtures involving Tl-204 beta rays with M150,H150 and Cs-137 . The ability to discriminate between the photons andthe beta fields is well demonstrated. However, there is nodiscrimination ability between these three photon fields amongthemselves.

From Table 2 it can be seen that the over-response of the dosimeterrelative to Cs-137 is approximately 20-25% with M150 and about 10% withH150. Since there is no clear discrimination among these three sources,whenever one of them is identified in a mixture the calibration factors(r_(abi)) are set to the average of the individual r_(ai) values forthose three photon fields. This procedure will overestimate the reporteddose from the M150 source by approximately 10-13% and underestimate theresponse to the Cs-137 and the H150 sources by approximately 10% and3-5%, respectively.

This bias, which is still well within acceptable tolerance levels suchas those established by DOELAP, represents the upper limit of theintentional "build-in" under- or over-response and will be fully appliedonly when those photon fields are pure. In a mixture, this bias isweighted down according to the relative contribution of the othercomponent (the value of N).

Although less pronounced, a similar situation may occur in some mixturesinvolving Tl-204 or depleted uranium. Examining Table 2, it can beascertained that the shallow dose responses of the two beta sources arewithin 20%. Again, if the average of the individual calibration factors(r_(a3) s) is used whenever Tl-204 or DU are identified in a mixture,the maximum "build-in" overestimation or underestimation of the DU orthe Tl-204 dose will be 10%, again well within acceptable DOELAPtolerance levels. For the calculations and results presented thepresence of a beta component was identified automatically by thealgorithm; however in some cases, the identity of the beta field (DU,Tl-204, etc.) was assigned manually for the purpose of the verificationof the superposition assumption. Similarly, for two mixtures involvingM30 (20 keV) and S60 (36 keV) photons, with Sr/Y-90 beta particles, S60was identified instead of M30 and Cs-137 instead of S60. In these twocases, the field identity was also assigned manually. Other than that,the radiation fields discrimination capabilities are clear particularlyfor photons and beta particles in a mixed photon beta field.

The above described procedures were applied to the raw data from 8different pure radiation fields involving 40 dosimeters that were usedto generate the calibration factors. In addition the same procedureswere applied to 19 different mixture irradiations using another group of95 dosimeters that were not used for the various calibrations. Theresults obtained from those dosimeters may serve as an independent testof the concepts and methodology herein described.

The mixture identification curve method was applied to all of the 135dosimeters that were involved. Each dosimeter was treated as if it wasexposed to a mixed field and the mixture components were identified.Furthermore, for each dosimeter, the relative contributions of eachfield, N and 1-N were calculated using Equation (23). If N was found tobe less than 0.15 or greater than 0.85, the reading was treated asresulting from exposure to a single source and the appropriate purefield calibration factor, r_(ai), was applied [this is equivalent tosetting N=0 or N=1 in Equations (29) and (30)]. The reason for imposingthis limitation on N is that even when the dosimeter is exposed to apure field, due to the unavoidable uncertainty in the measurement theprobability to get exactly 1 or exactly 0 is very small. Furthermore,according to DOELAP the mixture test categories are limited only to N'sin the range of 0.25 to 0.75.

                  TABLE 4                                                         ______________________________________                                        Comparison Between Delivered and Measured N values                            Mixture Components                                                            N             Relative Contribution of Field "a"                              Field "a"                                                                             Field "b" Delivered   Measured                                        ______________________________________                                        M30     Cs-137    0.696       0.674 + - 0.012                                 S60     Cs-137    0.491       0.441 + - 0.096                                 M30     Tl-204    0.483       0.561 + - 0.028                                 S60     Tl-204    0.466       0.458 + - 0.021                                 M150    Tl-204    0.416       0.515 + - 0.088                                 H150    Tl-204    0.415       0.464 + - 0.071                                 M30     Sr/Y-90   0.474       0.471 + - 0.043                                 S60     Sr/Y-90   0.458       0.470 + - 0.052                                 M150    Sr/Y-90   0.407       0.457 + - 0.011                                 H150    Sr/Y-90   0.406       0.410 + - 0.023                                 M30     Uranium   0.483       0.385 + - 0.061                                 S60     Uranium   0.466       0.500 + - 0.041                                 M150    Uranium   0.416       0.421 + - 0.026                                 H150    Uranium   0.415       0.438 + - 0.034                                 Cs-137  Tl-204    0.493       0.441 + - 0.075                                 Cs-137  Sr/Y-90   0.484       0.402 + - 0.056                                 Cs-137  Uranium   0.493       0.406 + - 0.087                                 ______________________________________                                    

To test the ability of the herein described system to use the dosimeterresponse not only to identify the radiation field components but also tomeasure the relative contribution of each component in a mixed field,the measured N values were compared to the N values as reported by thecalibration laboratory ("delivered") for the mixed fields. The resultsof this comparison, which are shown in Table 4, demonstrate goodagreement between the measured and the actual relative contribution ofthe various radiation fields. No comparison was made for the twomixtures of Cs-137 with M150 or with H150 since no meaningful N valuescan be computed due to the overlap of their MIC's as described in theprevious paragraph.

Some of the element ratios also were calculated using Equations (21) and(22), and were found to be in a good agreement with the experimentalresults as shown in Table 5. In both Tables 4 and 5, the uncertainty isrepresented by one standard deviation of results from five dosimeters.

                  TABLE 5                                                         ______________________________________                                        Example of Measured and Calculated Element Ratios                             Mixture Components                                                                              Element Ratios                                              Field "a"                                                                             Field "b"                                                                              Ratio    Calculated                                                                            Measured                                    ______________________________________                                        Cs-137  Sr/Y-90  L3/L1    1.012   1.013 + - 0.036                             Cs-137  M30      L3/L1    1.989   1.930 + - 0.058                             Cs-137  M30      L3/L2    3.095   2.916 + - 0.070                             ______________________________________                                    

The reported deep and shallow doses were calculated from the dosimeterresponses using Equations (25) to (32) and the calibration factors fromTable 2. The results for each category were compiled according to theguideline given in the Handbook For the Department of Energy LaboratoryAccreditation Program for Personnel Dosimetry Systems, DOE/EH-0026(1986), when the bias, B, is given by: ##EQU4## where P_(i) is thefractional difference between the reported and delivered absorbed doseor dose equivalent for the ith dosimeter, given by:

    P.sub.i =(Reported.sub.i - Deliveredi)/Delivered.sub.i     (34)

and the standard deviation is given by: ##EQU5## The |B|+S values forall of the 27 radiation fields involved are tabulated in Table 6 and arerepresented graphically in FIG. 8. For the pure fields except for thehigh energy photons, the bias is equal to zero since those values wereused for the calibration. For the high energy photons, the B valuesrepresent the previously described "build in" bias.

As demonstrated in FIG. 8, all the results are well within acceptabletolerance limits such as the current and planned DOELAP tolerancelimits.

                                      TABLE 6                                     __________________________________________________________________________    Summary of Verification Results                                                             SHALLOW DOSE DEEP DOSE                                          CATEGORY FIELD                                                                              B   S  B + S                                                                             L |B|                                                             S  |B| + S                                                          L                                      __________________________________________________________________________     1                                                                              III                                                                              M30      0.000                                                                             0.026                                                                            0.026                                                                             0.3                                                                             0.000                                                                             0.018                                                                            0.018                                                                              0.3                                     2                                                                              III                                                                              S60      0.000                                                                             0.024                                                                            0.024                                                                             0.3                                                                             0.000                                                                             0.018                                                                            0.018                                                                              0.3                                     3                                                                              III                                                                              M150     0.129                                                                             0.016                                                                            0.145                                                                             0.3                                                                             0.102                                                                             0.025                                                                            0.127                                                                              0.3                                     4                                                                              III                                                                              H150     -0.018                                                                            0.030                                                                            0.048                                                                             0.3                                                                             -0.006                                                                            0.061                                                                            0.067                                                                              0.3                                     5                                                                              IV Cs137    -0.111                                                                            0.017                                                                            0.128                                                                             0.3                                                                             -0.097                                                                            0.022                                                                            0.119                                                                              0.3                                     6                                                                              V  Tl204    0.000                                                                             0.034                                                                            0.034                                                                             0.3                                                                             0.000                                                                             0.000                                                                            0.000                                                                              N/A                                     7                                                                              V  Sr/Y90   0.000                                                                             0.017                                                                            0.107                                                                             0.3                                                                             0.000                                                                             0.000                                                                            0.000                                                                              N/A                                     8                                                                              VI DU       0.000                                                                             0.028                                                                            0.028                                                                             0.3                                                                             0.000                                                                             0.000                                                                            0.000                                                                              N/A                                     9                                                                              VII                                                                              M30 + Cs137                                                                            -0.069                                                                            0.022                                                                            0.091                                                                             0.4                                                                             -0.059                                                                            0.014                                                                            0.073                                                                              0.4                                    10                                                                              VII                                                                              S60 + Cs137                                                                            -0.034                                                                            0.025                                                                            0.059                                                                             0.4                                                                             -0.044                                                                            0.035                                                                            0.079                                                                              0.4                                    11                                                                              VII                                                                              M150 + Cs137                                                                           0.007                                                                             0.022                                                                            0.029                                                                             0.4                                                                             0.004                                                                             0.026                                                                            0.030                                                                              0.4                                    12                                                                              VII                                                                              H150 + Cs137                                                                           -0.096                                                                            0.019                                                                            0.115                                                                             0.4                                                                             -0.068                                                                            0.015                                                                            0.083                                                                              0.4                                    13                                                                              VII                                                                              M30 + Tl204                                                                            -0.030                                                                            0.040                                                                            0.070                                                                             0.4                                                                             0.018                                                                             0.022                                                                            0.040                                                                              0.4                                    14                                                                              VII                                                                              S60 + Tl204                                                                            0.043                                                                             0.023                                                                            0.066                                                                             0.4                                                                             -0.013                                                                            0.020                                                                            0.033                                                                              0.4                                    15                                                                              VII                                                                              M150 + Tl204                                                                           0.051                                                                             0.078                                                                            0.129                                                                             0.4                                                                             0.087                                                                             0.013                                                                            0.100                                                                              0.4                                    16                                                                              VII                                                                              H150 + Tl204                                                                           -0.008                                                                            0.053                                                                            0.061                                                                             0.4                                                                             -0.038                                                                            0.036                                                                            0.074                                                                              0.4                                    17                                                                              VII                                                                              M30 + Sr/Y90                                                                           -0.005                                                                            0.023                                                                            0.028                                                                             0.4                                                                             0.063                                                                             0.023                                                                            0.086                                                                              0.4                                    18                                                                              VII                                                                              S60 + Sr/Y90                                                                           0.031                                                                             0.016                                                                            0.047                                                                             0.4                                                                             0.013                                                                             0.023                                                                            0.036                                                                              0.4                                    19                                                                              VII                                                                              M150 + Sr/Y90                                                                          0.058                                                                             0.017                                                                            0.075                                                                             0.4                                                                             0.126                                                                             0.043                                                                            0.169                                                                              0.4                                    20                                                                              VII                                                                              H150 + Sr/Y90                                                                          -0.020                                                                            0.037                                                                            0.057                                                                             0.4                                                                             0.003                                                                             0.046                                                                            0.049                                                                              0.4                                    21                                                                              VII                                                                              M30 + DU 0.087                                                                             0.037                                                                            0.124                                                                             0.4                                                                             0.055                                                                             0.022                                                                            0.077                                                                              0.4                                    22                                                                              VII                                                                              S60 + DU 0.004                                                                             0.057                                                                            0.061                                                                             0.4                                                                             0.017                                                                             0.017                                                                            0.034                                                                              0.4                                    23                                                                              VII                                                                              M150 + DU                                                                              0.154                                                                             0.036                                                                            0.190                                                                             0.4                                                                             0.124                                                                             0.026                                                                            0.150                                                                              0.4                                    24                                                                              VII                                                                              H150 + DU                                                                              0.032                                                                             0.016                                                                            0.048                                                                             0.4                                                                             0.019                                                                             0.031                                                                            0.050                                                                              0.4                                    25                                                                              VII                                                                              Cs137 + Tl204                                                                          -0.074                                                                            0.030                                                                            0.104                                                                             0.4                                                                             -0.114                                                                            0.018                                                                            0.132                                                                              0.4                                    26                                                                              VII                                                                              Cs137 + Sr/Y90                                                                         -0.056                                                                            0.034                                                                            0.090                                                                             0.4                                                                             -0.073                                                                            0.026                                                                            0.099                                                                              0.4                                    27                                                                              VII                                                                              Cs137 + DU                                                                             -0.045                                                                            0.068                                                                            0.113                                                                             0.4                                                                             -0.083                                                                            0.013                                                                            0.096                                                                              0.4                                    __________________________________________________________________________

It is here noted that the above referred to TL measurements wereperformed using a System Model 8800 automatic TLD card reader sold byHarshaw Crystal & Electronic Products, Solon, Ohio. The Model 8800 TLDcard reader utilizes a non-contact heating technique based onpre-purified hot nitrogen gas. The Model 8800 TLD card reader employs aprogrammable, precisely controlled linear time temperature profile forreproducible heating of the dosimeter elements. This is desirablebecause the amount of radiation induced TL is dependent on the thermalhistory of the material as well as on the heating rate during readout. Afully controlled heating cycle is therefore important especially for lowdose measurements. However, its most important advantage is thepossibility for continuous control of the heating cycle using variousfeed-back techniques.

The TL signal is accumulated simultaneously from the four TL elements ina card via a charge integration technique using four thermoelectricallycooled photomultipliers. Glow curves were recorded to a maximumtemperature of 300 degrees C at a heating rate of 25 degrees C/sec. Nohigh temperature annealing was applied and the preparation of thedosimeters prior to irradiation consisted of subjecting the dosimeter toone readout cycle through the reader. The residual TL signals using thisreader anneal technique were found to be less than 0.5% at the Sr/Y-90rad level. All the irradiations to determine the element correctioncoefficients and the reader calibration factor were performed in thereader using an internal shielded 0.5 mCi Sr/Y-90 irradiator with anautomatic shutter mechanism that can be timed in units of 0.1 secondwhich corresponds to approximately to 1.2 mrad. The reproducibility ofthe internal irradiator was found to be better than 1% (one standarddeviation of 10 repeatable irradiations) at the Sr/Y 500 mrad level.Furthermore, no significant changes in the glow curve structure wereobserved due to repeated irradiation and readout.

The calibration irradiations were carried out by the calibrationlaboratory. For the photon irradiations, the calibrations performed bythe calibration laboratory were in agreement with the calibrationsperformed by NBS between 0.6% and -4.8%. The overall uncertainty in thebeta irradiations was estimated to be ±3%.

A more general and improved methodology will now be described, themethodology providing for determination of dose resulting from pure andmixed fields composed of a variety of sources including beta, photonsand neutrons. Summarized in Table 7 are a variety of sources which maybe used to perform a series of calibration irradiations according to theabove procedure. Calibration irradiations normally would be performedfor all possible radiation fields of interest, i.e., typically thosetypes of fields to which personnel may be exposed at a particularfacility or which, in general, are to be monitored.

                  TABLE 7                                                         ______________________________________                                        Calibration Irradiations                                                      Radiation Field         Energy                                                ______________________________________                                         1   x-ray Monoenergetic - K16                                                                             16 keV                                            2   x-ray NBS filtered Technique - M30                                                                    20 keV                                            3   x-ray NBS filtered Technique - S60                                                                    36 keV                                            4   x-ray monoenergetic - K59                                                                             59 keV                                            5   x-ray NBS filtered Technique - M150                                                                   70 keV                                            6   x-ray NBS filtered Technique - H150                                                                   120 keV                                           7   Gamma Cs-137            662 keV                                           8   Beta (Point geometry) Tl-204                                                                          760 keV (max)                                     9   Beta (Point geometry) Sr/Y-90                                                                        2300 keV (max)                                    10   Beta (Slab geometry) Uranium                                                                         2300 keV (max)                                    11   Neuron - Cf-252        Moderated                                         12   Neutron - Cf-252       Unmoderated                                       ______________________________________                                    

The dosimetry method comprises the step of using the above-describeddosimeter 10 (FIGS. 1-3) to monitor exposure to a mixed radiation fieldcomposed of photons, beta particles and/or neutrons. The radiateddosimeter is heated to obtain thermoluminescence emission of the TLelements. The emission is then quantitatively related to the radiationincident on the dosimeter according to the following procedure, which isdiscussed in the form of a decision tree as is particularly desirablefor computer implementation. The following decision tree is applied toreport absorbed dose in terms of dose equivalent.

Step 1

As an initial step the reported TL responses (L1, L2 and L3) of the TLelements are checked to see if they correspond to a physically realisticsituation. More particularly, the element ratios L3/L1 and L3/L2 aretested to determine if their values correspond to a physically realisticsituation. If either of these ratios is smaller than 0.7 or the ratioL3/L1 is greater than 11 and for the same dosimeter, the ratio L3/L2 issmaller than 4, there is a possibility of an error in the system and thereading should be identified for questioning.

Step 2

The next step is to determine whether or not there is a neutroncomponent and to evaluate its associated TL signal. For a mixed photonneutron field, TL element 17 in position 1 (TLD-700) and TL element 20in position 4 (TLD-600) are used to determine the neutron dosepreferably via known albedo methodology. If TL elements 17 and 20 werecovered by equal thickness filters like in known symmetric neutrondosimeters, the neutron contribution to the total TL signal could beeasily determined from simple subtraction of the TL responses (L4-L1) ofTL elements 17 and 20. However, in the instant dosimeter the filterthicknesses covering TL element 17 (1000 mg/cm²) is greater than thatcovering TL element 20 (300 mg/cm²) to enable estimation of dose to thelens of the eye as well as to measure neutron dose. Consequently simplesubtraction of the TL responses (L4-L1) to determine the neutroncontribution to the total TL signal may over-estimate the neutron dose,as a low energy photon component will be attenuated more in filter 13 atposition 1 than in filter at position 4. To overcome this limitation,the responses of the TL elements 17 and 20 are relatively weighted.Preferably, the response of TL element 17 is multiplied by a weightingfunction F(x_(p)) which depends on the photon energy. Photon energy maybe represented as the variable x_(p), given by: x_(p) =log (L1/L2) andF(x_(p)) is herein determined experimentally using mixed fields such asthose composed of moderated 252_(Cf) neutrons and photons of variousenergies. Experimental data is shown in FIG. 10, in which the solid lineis represented by an interpolative polynomial:

    F(xp)=1+3.5x.sub.p -1.9(x.sub.p).sup.2                     (36)

The TL signal associated with the response to neutrons, N_(TL) is thengiven by:

    N.sub.TL =L4-F(x.sub.p)L1                                  (37)

The quantity N_(TL) can be used to indicate the existence of neutrons inthe mixed field such as by computing the ratio N_(TL) /L1, which is theratio between the neutron and the photon induced TL signals. Forexample, when N_(TL) /L1 >0.2 and L4/L3>1.2, this is an indication thatthere is a neutron component included in the induced TL signal whichcontributes at least 20% of the TL response associated with photons. Thecondition on L4/L3 is used to distinguish between neutron and betainduced TL signal on TL element 4.

The neutron dose, Hn, may be represented by:

    H.sub.n =N.sub.TL /K.sub.nd                                (38)

when K_(nd) is the response of the dosimeter to the neutron field ofinterest relative to its response to ¹³⁷ Cs photon fields.

Step 3

A determination is made to see if the incident radiation is almostexclusively beta particles. When the ratio L3/L1 is greater than 15,there are almost exclusively beta particles. The shallow dosecalibration factor herein is determined experimentally. For example, theshallow dose calibration factor may be given by:

    r.sub.s =80300-13500 log (L3/L2)                           (39)

Equation (39) was determined based on experimental data collected withpoint beta sources in the energy range between ⁹⁰ Sr/Y and ²⁰⁴ Tl. Theunits of r_(s) in Equation (39) at this point are gU/Sv (gU defined forthe above identified local reference source).

Step 4

When both ratios L3/L2 and L3/L1 are in the range of 0.7 to 1.1, thereare preferably only intermediate and high energy photons in the field.The deep and shallow dose calibration factors r_(d) and r_(s) arepreferably calculated as the average response of the dosimeter to M150,H100 and ¹³⁷ Cs, which results in 73100 and 75100, respectively (units:gU/Sv). This averaging process may result in under-estimation of ¹³⁷ Csdoses by approximately 10% and over-estimation of M150 doses byapproximately the same amount.

Step 5

If it has been determined that there are no neutrons in the field, andthat the field is not a "beta only" or a "beta and intermediate and highenergy photons" only field, a check is made to determine if the fieldconsists of a mixture of beta and photon fields. If L1/L2 is less than1.1, L2/L3 is less than 0.8, and L4/L2 is less than 1.3, a betacomponent is mixed with intermediate and high energy photons. The deepdose calibration factor is then computed as in Step 4. On the otherhand, the shallow dose calibration factor is calculated assuming thatthe response of TL element 19 is the superposition of its response tophoton and beta:

    r.sub.s =75100N+60700(1-N)                                 (40)

where N is given by:

    N=(597-80x)/(911x-377)                                     (41)

where x is the ratio L3/L2.

Step 6

When the ratio L3/L4 is greater than 1.1 and the ratio L1/L2 is lessthan 3, a beta component is mixed with low energy photons, and the deepand shallow dose calibration factors R_(d) and R_(s) respectively aregiven by:

    r.sub.d =89100 if 1<L1/L2<1.8,                             (42a)

    r.sub.d =71800 if 1.8<L1/L2<5.5                            (42b)

and

    r.sub.s =87400-17000x'                                     (42c)

where x'=log (L3/L2).

Step 7

If it is determined that there is no beta component, the dosecalibration factors are determined for photons having various energies,as follows:

    r.sub.d =a.sub.d e.sup.bdx                                 (43a)

    r.sub.d =a.sub.s e.sup.bsx                                 (43b)

where X L3/L2 and the coefficients a_(i), b_(i), i=1, 3 are given inTable 8.

                  TABLE 8                                                         ______________________________________                                        L3/L2      a1      b1         a3    b3                                        ______________________________________                                        0.9-1.5    38000   0.63       34700 0.75                                       1.5-10.5  57200   0.036      73800 0.012                                     10.5-15    98900   0.0        73800 0.012                                     ______________________________________                                    

Step 8

After r_(d) and r_(s) have been determined, the dose is computed asfollows:

    Hs=L3/(r.sub.s Ks)                                         (44a)

    Hd=L1/(r.sub.d Kd)                                         (44b)

where Hs and Hd are the deep and shallow dose equivalent respectively inunits of Sv; Ks and Kd are the local units to ¹³⁷ Cs dose equivalentconversion factors for shallow and deep dose respectively (which usuallywill be different from site to site, depending on the local referencesource being used), given by:

    K.sub.d =[C1/D]/66000                                      (45a)

    K.sub.s =[C3/D]/66800                                      (45b)

when C1 and C3 are the responses (in local gU) units of the TL elementsat positions 1 and 2, respectively, when a quantity of D (Deep orShallow) dose is being delivered by a calibrated ¹³⁷ Cs source at thecalibration laboratory.

Step 9

For dose levels greater than 1 Gy, corrections to account for thesupralinearity of LiF:Mg,Ti are applied. The supralinearity correctionis given by the TL dose response curve, f(D):

    f(D)=[TL(D)/D]/[TL(D.sub.o)/D.sub.o ]                      (46)

where TL(D) is the TL signal corresponding to dose D and TL(D_(o)) isthe TL signal corresponding to D_(o), D_(o) being in the linear regionof the TL dose response curve. The dose response curve for TL element 17has been measured over the dose range of 1 to 6 Gy, resulting in thefollowing dose response curve.

    ______________________________________                                        Range        Correction (f(D) = )                                             ______________________________________                                        D ≦ 1 Gy                                                                            1.0                                                              1 < D ≦ 2 Gy                                                                        1.05                                                             2 < D ≦ 4 Gy                                                                        log (100D) - 1.2                                                 4 < D < 6 Gy 1 + 0.03[log(100D)] + 0.005[log(100D).sup.2 ]                    ______________________________________                                    

Step 10

To account for the fading effects, a fading correction function for theentire glow curve has been determined experimentally:

    F(t)=e.sup.-a(t-8)                                         (48)

where t=T/2 and a=0.0033 day-1. T is the lapsed time in days betweenpreparation (reader anneal) and readout. The fading correction function(48) was determined experimentally and is limited to dosimetry issueperiods (T) up to 90 days. The measured TL signal included thecontribution of the low temperature peaks 2 and 3, and no pre-heat orany other fading reduction techniques were applied. The fadingcorrection given by Equation (17) represents the maximum amount offading expected for LiF:Mg,Ti. The actual fading corrections needed inevery case will depend on the methods used, if any, to remove the lowtemperature peaks; or, if the sum of peaks 4 and 5 is used, no fadingcorrection is required. Both the supralinearity correction factor f(D)and the fading correction factor F(t), are applied as divisors.

The above procedure was applied to a variety of pure and mixed radiationfields. The results for each category were compiled using the guidelinegiven in the above referenced Handbook for the Department of EnergyLaboratory Accreditation Program for Personnel Dosimetry Systems, whenthe "Bias", B, and the standard deviation, S, are given by Equations(33) and (35), respectively. Typical |B|+S values for radiation fieldsinvolving various photon, beta and mixtures are represented graphicallyin FIG. 11 and compared to the current (solid line) and expected (dottedline) DOELAP tolerance levels. Similar test results were obtained usingfour irradiations involving neutrons or mono-energetic x-ray beams, assummarized in Table 9 (next page).

In spite of the variety of possible irradiation conditions that mightexist in the field, the errors in estimating doses to personnel fromexternal sources can be minimized, providing that appropriate dosimetrictools are being used and enough attention is paid to properly use,characterize and calibrate those tools. Important factors to beconsidered are: (1) adequate filtration and the use of tissue equivalentor nearly tissue equivalent TL detectors, (2) establishment of local orgeneric. units (gU) using a stable local reference source and stablereference dosimeters for NBS traceability, (3) performance of detailedcalibration using variety of NBS calibrated pure fields to establish thevarious calibration constant a_(i) and r_(ai), and (4) frequent readercalibration using the local reference source and tracking any possiblechanges in the reader condition.

                  TABLE 9                                                         ______________________________________                                                    Deep Dose                                                         Field         B           S      |B| + S                    ______________________________________                                        Cs-137 + Cf-252U                                                                            -0.011      0.026  0.037                                        M30 + Cf-252U -0.042      0.012  0.054                                        S60 + Cf-252U 0.074       0.014  0.089                                        M150 + Cf-252U                                                                              0.105       0.028  0.133                                        H150 + Cf-252U                                                                              0.031       0.028  0.059                                        K16 + Cf-252U -0.037      0.032  0.068                                        Cf-252U + None                                                                              -0.015      0.041  0.056                                        K16           0.008       0.010  0.018                                        K59           -0.097      0.027  0.124                                        K59 + Cf-252M 0.204       0.037  0.241                                        Cs-137 + Cf-252M                                                                            -0.044      0.017  0.061                                        M30 + Cf-252M 0.015       0.014  0.028                                        S60 + Cf-252M 0.062       0.012  0.074                                        M150 + Cf-252M                                                                              0.097       0.031  0.129                                        H150 + Cf-252M                                                                              0.020       0.023  0.044                                        K16 + Cf-252M 0.100       0.022  0.123                                        Cf-252M + None                                                                              -0.010      0.030  0.040                                        K59 + Cf-252U -0.172      0.021  0.193                                        ______________________________________                                    

The above described methodology, to the extent permitted, preferably iscarried out by programs and sub-routines of an appropriately programmedcomputer or processor means. Accordingly, a personnel dosimetry systemaccording to the invention comprises, as diagrammatically shown in FIG.9, an appropriately calibrated TLD reader 10 and a computer 12programmed to operate on the TL response data acquired by the TLD readerto identify radiation field mixture using correlation functions orcurves and to determine dose equivalent.

As will be appreciated, the above described methodology and system maybe modified as needed to accommodate various radiation fields ofinterest for radiation monitoring. In addition, the above describedmethodology may be adapted by those skilled in the art to differenttypes of dosimeters other than that above described, although the abovedescribed dosimeter is a preferred dosimeter. For example, the neutrondose estimation procedure can be generally applied to nonsymmetricneutron dosimeters, wherein the TL elements which provide for highenergy photon and neutron dose estimation are assymmetrically filtered.Still other features of the invention have general application tovarious types of dosimeters and dosimetry systems.

Although the invention has been shown and described with respect to apreferred embodiment, it is obvious that equivalent alterations andmodifications will occur to others skilled in the art upon the readingand understanding of this specification. The present invention includesall such equivalent alterations and modifications, and is limited onlyby the scope of the appended claims.

What is claimed is:
 1. A dosimetry method comprisingusing a dosimeter tomonitor exposure to a mixed radiation field composed of photons andneutrons, the dosimeter including first and second elements sensitive tophotons and insensitive to neutrons and a third element sensitive tophotons and neutrons, and first and second filters for said first andsecond elements having different photon attenuation characteristics anda third filter for said third element having a photon attenuationcharacteristic different from the photon attenuation characteristic ofthe first filter; heating the radiated dosimeter to obtainthermoluminescence emission of the elements; and quantitatively relatingthe emission to the radiation incident on the dosimeter, saidquantitatively relating including determining a neutron component ofincident radiation by taking the difference between the response of thefirst and third elements with one of the responses being weighted as afunction of photon energy.
 2. A dosimetry method as set forth in claim1, wherein the weighting function is determined experimentally usingmixed fields of neutrons and photons of various energies.
 3. A dosimetrymethod as set forth in claim 1, wherein the photon energy is based onthe ratios of the TL responses of said first and second elements.
 4. Adosimetry system comprisinga dosimeter including first and secondelements sensitive to photons and insensitive to neutrons and a thirdelement sensitive to photons and neutrons, and first and second filtersfor said first and second elements having different photon attenuationcharacteristics and a third filter for said third element having aphoton attenuation characteristic different from the photon attenuationcharacteristic of the first filter; means for heating said dosimeter,after exposure to a mixed radiation field composed of photons andneutrons, to obtain thermoluminescence emission of the elements; andcomputer means for quantitatively relating the emission to the radiationincident on the dosimeter, said quantitatively relating means includingmeans for determining a neutron component of incident radiation bytaking the difference between the response of the first and thirdelements with one of the responses being weighted as a function ofphoton energy.
 5. A dosimetry system as set forth in claim 4, whereinthe weighting function is determined experimentally using mixed fieldsof neutrons and photons of various energies.
 6. A dosimetry system asset forth in claim 4, wherein the photon energy is based on the ratio ofthe TL responses of said first and second elements.